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One of two things must be done for the growth of thick films to be possible; either the deposition time or rate must be increased. An attempt was made at thick film growth by increasing the deposition rate (by increasing the repetition rate of the laser), but the quality of the films was found to be heavily reduced, so the use of longer deposition times was chosen instead. The side effects that arise due to the use of long deposition times have to be managed so that the quality of films is not significantly affected.

Long deposition times require the target position to be changed regularly or reconditioned (polished) to prevent loss in quality as a result of poor target surface condition. It was found that the quality of films deposited over a period of more than two hours using the same target position decreased significantly, so growth runs of no more than one and a half hours were used. When repositioning or reconditioning the target, the chamber must be opened and care must be taken whilst this procedure is carried out to prevent intermediate layer contamination or disturbance of the substrate. Figures 4.2.4 and 4.2.5 show SEM pictures of the surface of a YbAG target crystal used for 1.5 hours and 9 hours respectively.

Magnification = 55 × 1 mm Magnification = 300 × 100 m

Figure 4.2.4: SEM image of a YbAG crystal target that has been used for 1.5 hours of deposition.

Magnification = 55 × 1 mm Magnification = 300 × 100 m

Figure 4.2.5: SEM image of a YbAG crystal target that has been used for 9 hours of deposition.

The difference in target damage can be seen clearly and the roughness that can be seen in figure 4.2.4 is known to increase the occurrence of particulates significantly. Degradation of the target surface quality means that rods (rotated so that the surface is ablated in a spiral pattern) are impractical targets because it is difficult to recondition the target surface. Another consequence of long deposition times is the gradual drift of deposition conditions, and it was found that deposition conditions must be constant over separate growth runs, or else the layers won't grow epitaxially. Evidence of non-epitaxial layers was observed under a microscope at the polishing stage of one sample in the form of a line separating the two layers. Figure 4.2.6 is an optical microscope image that shows an example of a film with a fault line separating two growth run layers. The film was deposited in eight growth runs, and from knowledge of the deposition rate, the fault line appears to be between the fifth and sixth growth runs. Cracks thought to be due to thermal expansion mismatch (to be discussed later) can also be seen in figure 4.2.6.

20 m Magnification = 50 × Nd:GGG waveguide YAG substrate YAG capping layer mounting wax fault line due to

non-epitaxial growth cracks

Figure 4.2.6: Optical microscope image of a sample with a fault line separating non-epitaxial layers.

Laser port windows can get coated, changing the laser power that enters the chamber throughout the depositions and causing a drift in film characteristics throughout thickness. Metal pipes were placed in the chamber to shield port windows and prevent this effect from becoming significant. The excimer laser gas mix gradually degrades over time and this can affect the beam shape, distribution and output power. The excimer gases must be changed regularly to avoid this causing different conditions in separate growth runs. General cleanliness in and around the chamber is more important when performing multiple growth runs so that no localised layer deficiencies occur.

The maximum thickness of films that could be successfully refined into devices by polishing was limited by cracking, thought to be due to thermal expansion mismatch. Films of thickness up to 135 μm were deposited in multiple growth runs, and they appeared to be stable until polishing was performed. We therefore surmise that the stresses and/or vibrations involved in the polishing process are a catalyst for cracking. The thickest film

(grown on a 0.5 mm thick substrate) to survive the polishing process was 40 μm. Figure 4.2.7 is an optical microscope image of a sample (with a 0.5 mm thick substrate) where the substrate has several cracks and will completely fall apart if demounted.

Magnification = 5 × packing layer YAG substrate Nd:GGG waveguide YAG capping layer packing layer cracks 200 m

Figure 4.2.7: Optical microscope image of a sample with cracks in the substrate. We changed from using 0.5 mm thick substrates to 1 mm thick substrates in an attempt at preventing the cracking from happening and this move was partially successful; the samples no longer shattered completely into pieces, but the film edges perpendicular to the ones being polished did persist in cracking away and it wasn't possible to polish all four sides for side-pumping. It was possible however to polish two opposing parallel sides, allowing the waveguides to be end-pumped. Figure 4.2.8 is an optical microscope image of a sample (with a 1 mm thick substrate) where the edges perpendicular to the ones being polished have cracks running down them and will come apart from the sample if demounted.

packing

layer substrateYAG waveguideNd:GGG cracks

Magnification = 20 × 50 m

Figure 4.2.8: Optical microscope image of a sample with cracks in the edge perpendicular to the one being polished.